1. Field of the Invention
The present invention relates to a method for removing impurities from a crude nickel sulfate material containing impurities such as ammonia, sodium, cobalt, iron , copper, zinc, calcium, magnesium and others, the impurities to obtain a highly-purified nickel sulfate solution while optionally recovering cobalt.
2. Description of the Prior Arts
Nickel sulfate is industrially used not only in ordinary electroplating but also in electroless nickel plating for producing computer hard discs. In addition, nickel sulfate has been recently used as the nickel material for secondary batteries.
For some of those uses, the amount of impurities such as ammonia, sodium, cobalt, iron, zinc, copper, calcium, magnesium and others that may exist in nickel sulfate must be reduced as much, as possible. Purification of crude nickel sulfate is generally effected through solvent extraction. For this, in general, employed is a method of extracting the impurities existing in a crude nickel sulfate solution into an acid extractant, for example, an organic, phosphorus-containing acid extractant such as a phosphric acid ester or a phosphric acid ester to remove the impurities, thereby giving a purified nickel sulfate solution; or a method of extracting nickel from a crude nickel sulfate solution into such an extractant followed by stripping the nickel-loaded organic phase with sulfuric acid to obtain a purified nickel sulfate solution. Those extraction methods using such an acid extractant require a neutralizer such as sodium hydroxide or ammonia, as hydrogen ions are released therein during extraction of the impurities from the crude nickel sulfate solution into the extractant or during extraction of nickel from the solution into the extractant.
For example, in the former method of extracting the impurities from the crude nickel sulfate solution into the acid extractant used therein, the pH of the extraction system is so controlled that the impurities of cobalt, calcium, iron, zinc, copper and others, for which the extraction pH is lower than that for nickel, could be selectively extracted into the extractant and separated and removed from the nickel sulfate solution to give a purified nickel sulfate solution. In this, however, the neutralizer necessary for the extraction reaction is problematic in that Na.sup.+ or NH.sub.4.sup.+ ions released from it move into the nickel sulfate solution to contaminate the purified nickel sulfate solution.
On the other hand, in the latter method where nickel is selectively extracted from a crude nickel sulfate solution containing such impurities into the acid extractant used therein, impurity elements to be extracted at a pH lower than that for the nickel extraction shall also be extracted into the acid extractant along with nickel. In addition, in this, the nickel extraction will be inevitably accompanied by some sodium and ammonia extraction. Moreover, as so mentioned above, this method inevitably requires a neutralizer for controlling the pH of the extraction system, resulting in that the organic phase as finally separated in the method shall contain all impurities. In general, the organic phase is stripped with sulfuric acid to recover nickel from it, but it is difficult to remove all those impurity elements through the stripping.
Accordingly, the nickel loaded organic phase is strongly scrubbed to remove sodium and ammonia therefrom. To remove the other impurities, nickel sulfate obtained as a result of the stripping with sulfuric acid is further again extracted with different extractants. Anyhow, such repurification must be repeated to extract and remove the respective impurities. Therefore, the conventional methods are extremely uneconomical in that the scrubbing of the nickel-loaded organic phase requires a large amount of scrub solution and requires additional treatment of the scrub raffinate, while being accompanied by nickel loss, and that the additional extraction to remove cobalt and other impurities from the phase requires different solvent extraction devices. On the other hand, a crude nickel sulfate solution to be purified often contains a relatively large amount of cobalt, and cobalt that shall remain in the organic phase to be separated as a result of extraction of the solution is a valuable metal. Economically, therefore, efficient recovery of cobalt is desired.